The present invention relates to the treatment of flyash and sewage sludge to produce a lightweight aggregate. More specifically, the present invention relates to a method of processing a pelletized agglomerate of sewage sludge and flyash in a rotary kiln by the introduction of port air into the bed of pelletized agglomerate in the rotary kiln.
Flyash is a particulate by-product produced from the burning of coal, and particularly powdered bituminous coal. Coal burning power plants typically produce very large quantities of flyash that must be disposed of in an environmentally acceptable manner. In recent years, the cost of placing flyash in a landfill has significantly increased such that the option of treating the flyash to form a commercially desirable product has become a more and more commercially viable prospect. The resultant product formed from treated flyash is typically used as a lightweight aggregate in the formation of concrete structures. In the process of treating the flyash, a typical step involves heating a flyash mixture in a rotary kiln.
Sewage sludge is the by-product of the treatment of waste water. Sludge is the settled solids accumulated and subsequently separated from the liquid stream during various phases of the waste water treatment process. The sludge may be from primary or secondary settling, or may be waste-activated sludge. The sludge may be raw sludge, digested sludge or de-watered sludge. The characteristics of the sludge will vary depending upon the stage of treatment from which it is drawn and also depending on whether it has received treatment, such as by digestion. However, a common characteristic of the sludge is that it contains significant organic materials.
Sludge has been generally disposed of by incineration followed by land disposal of the inert ash or by lagooning, landfilling, spreading on land for fertilizer for soil conditioning and ocean dumping where permitted. As with flyash, sewage sludge presents a considerable problem of disposal in an economical and environmentally sound manner.
In accordance with Nechvatal et al. U.S. Pat. Nos. 5,057,009 and 5,342,442, the disclosures of which are incorporated herein by reference, a method for producing a lightweight aggregate by treating a combination of flyash and sewage sludge is known. In accordance with the process shown in these two patents, sewage sludge and flyash is agglomerated into pellets which are coated and dried. The dried pellets are introduced into a rotary kiln in a direction that is co-current with the flow of fuel and air through the kiln. The pellets within the kiln experience complete calcination and various degrees of pyrolizing and sintering to produce a nodular material having a low density but with a hard, porous structure. The resultant nodular product, after cooling, can be used as a lightweight aggregate for concretes, masonry, or insulation purposes, can be used for other commercial purposes, or can otherwise be disposed of in an environmentally sound and economical manner.
Although the process taught in the Nechvatal et al. patents has proved to be an effective method of producing a lightweight aggregate from flyash and sewage sludge, it is an object of the present invention to provide an improved rotary kiln that produces a better product using less external fuel during the passage of the pelletized feed stock through the rotary kiln. It is an additional object of the invention to increase the combustion of the material fuel in the pelletized feed stock near the infeed end of the rotary kiln such that the material fuel contained within the feed stock increases the temperature within the rotary kiln. It is an additional object of the invention to provide a method of forming lightweight aggregate from flyash by mixing the flyash with sewage sludge that provides a major portion of the fuel value needed in a rotary kiln treatment of the flyash.